How to Box Corners

Editor: Liz Johnson

Tuesday, 30 August 2016 1:00

Okay - true confession time. In school, I was a theater rat... always in plays and musicals, always taking artsy-fartsy classes, including "How To Mime" or, as I remember it, "How To Pretend You're Stuck In A Box And Look Foolish Doing It." Unless you're Marcel Marceau, you look really silly doing mime. So... no mime today. But, we are still making a box. In particular, a boxed corner. This is a sewing technique everyone should have in her/his arsenal. The boxed corner creates space in something that would otherwise be flat. For example, in a bag, you'll have a lot more room to stash your stuff if you create boxed corners. Basically, any sewn corner can be turned into a boxed corner with a few simple steps. We show you the two most common methods.

If you sew two squares together, then turn them right side out, the square is still flat. If you were to stuff it with some filler, it would make a lovely knife-edge pillow. If you want to turn your flat square into something with more dimension, you add boxed corners.

We love the box corner here at S4H. Below are just a few projects made more appealing and functional thanks to their boxed corners.

For this tutorial, we've used a light fabric with contrasting thread so you can clearly see the steps of our techniques. You would use the fabric of your choice with coordinating thread.

In this example, we're assuming you’re working on a project that has sewn corners and is shaped like a square or rectangle. If you’re following a pattern or tutorial, the boxed corners (as well as the exact measurements) will be indicated.

Place the fabric right side together.

Using a straight stitch, and the indicated seam allowance, sew the side and bottom seams, pivoting at each corner. We used a ½" seam allowance.

With the sewn fabric still right sides together, use both hands to pinch and pull apart the corner. As you keep pulling, the fabric will begin to make a little peak with the corner point at the top and the seam lines running down the middle of the front and the back. Align these side and bottom seams. Place a pin in the matched seams to hold them together. It is very important that you exactly match the seams; that is what will make the intersecting lines of your finished corner look good.

Mark the depth of the boxed corner with a fabric marking pen or pencil and a straight ruler, positioning your ruler so the depth is measured from side to side at the base of the "peak." Our boxed corner depth is 3” (more on how to determine measurements below). Slide the ruler down from the peak until you reach the point at which your "triangle" is at the appropriate width (3" in our sample). Draw a horizonal line at this measurement.

Another measuring option is to measure vertically from the corner point of your seam (the actual end point of the seam – not the tip of the fabric) down along the seam line. In this case, you measure HALF the width of your finished corner – or 1½" in our sample. It's a bit harder to tell in the photo below, but from the point of the seam to the ruler is 1½" – look at the grid lines on the cutting mat, which are 1" squares. Draw a horizontal line at this measurement.

Sew across the peak on the drawn line. Be sure to backstitch at the beginning and end of the seam.NOTE: Depending on the project you’re sewing, you may want sew across two or three times to reinforce for added strength at corner. This would be an especially good practice for a bag.

Trim away the peak to ¼" - ½" from the line of stitching.

Gently pull apart the corner at the seamline to see the boxed corner take shape.

Turn the fabric right side out to really see what your finished boxed corner looks like. Lovely!

Cut-out box corner

As with many sewing techniques, there’s an alternate way to create a boxed corner. Some people prefer to cut a square from each corner, then bring the seams together to sew the corner. You get the exact same finish as above, just in a different way. You may find this to be a better approach to sewing a boxed corner with heavier-weight fabrics.

Place the fabric right sides together on a flat surface.

Measure, mark and then cut equal size squares from each corner. In our example, we wanted a 4" finished corner and so cut out our corner squares at HALF that size or 2" x 2" squares (if you are using a pattern, you should be given your cut dimensions, but there are additional notes on measuring below).

Using a straight stitch, and the indicated seam allowance, sew the side and bottom (or adjacent) seams separately (because you just cut away the corner you would have pivoted at!). NOTE: You can also sew the side and bottom seams first, pivoting at the corner (as above) and then cut out the corner. You just need to be sure of all your sizing. It doesn't matter that you are cutting into the seam as it will be secured again when the corner itself is stitched.

In the same manner as the basic box corner above, carefully match the side seam with the bottom or adjacent seam. Pin in place.

Sew along raw edge of the corner, using a straight stitch and the indicated seam allowance. In our example, our side and bottom seams were ½" so our seam allowance across the corner should also be ½". Be sure to backstitch at the beginning and end of the seam.NOTE: Interestingly enough, you will sometimes see pockets shaped like this (with the two bottom corners cut out) except they use just one layer of fabric. Think about a cell phone pocket inside a bag; you need depth in the pocket for the phone... solution: boxed bottom corners.

Measurements

If you’re not following a pattern or tutorial that tells you the proper measurements, you will need to do some calculating to determine the size (or depth) of your boxed corner. It’s recommended you start with a shorter distance and go from there; you can always increase the size of box corner (especially if you’ve already trimmed away the fabric), but ya can't go smaller after you've made your cuts. As always, we recommend testing any new technique on scraps prior to starting on your project.

Basic box corner

With a basic box corner, the distance from one folded edge to the other at the base of the peak is the depth of the box corner. Remember, in our example above, we sewed 3” across the peak from fold to fold, which yielded a 3" box corner. Or from the top fold of the peak, you can measure HALF the width of what you want your finished corner to be.

The farther away from the peak that you draw your line and sew, the deeper the box corner will be. Using a ruler to mark the stitch line will help you determine the depth, plus it helps you stay consistent on the other corner(s).

Cutout box corner

When you plan to use the cutout box corner method, you need to pre-determine the size of the box corner. The most important thing to remember is: the size of the square you cut away is HALF the size of your finished box corner. In our example above, we cut out a 2" x 2" square, which resulted in a 4" box corner.

The other detail to remember is seam allowance. You want to use the exact same seam allowance across the box corner seam as was used for your side and bottom (or adjacent) seam allowance.

As always, accuracy in cutting is key so the box corners are consistent in size.

A few additional helpful notes to remember about box corners

The deeper or larger the box seam, the shorter the project.

The smaller or shorter the box seam, the less depth you will have overall.

If you’re making a bag, remember that you will need to create identical box corners on your lining.

Contributors

Sample Creation and Instructional Outline: Jodi Kelly

Section:

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Comments (24)

Lynda Hardin-Poston, PhD said:

Thank you so much for your very simple and easy instructions on TWO ways to make a box corner. I have been sewing since I was 4 years old and I have always been intimidated by box corners. I finally broke down and thought I would try. The most that could happen is I would have to start over, so I found a YouTube video that completely dumbfounded me and I thought I would NEVER figure that out. So I looked again and found this one that shows pictures and very easy instructions. I read them and said, "I can do that!!" As a young woman I could completely break down a car engine and transmission and rebuild them, this can't be any harder than that. So I dug in and they came out perfect the first try!!! Thank you so much for helping me overcome something that has had me terrified since I was a child. You made it sound so simple, and it WAS!! Thanks again. Now on to the next challenge. LOL

I followed the formula for the pieces I cut but I've ended up with 2 extra inches of height after boxing. Would it make more sense to add half the depth of the bag since only half the depth is actually taken away from the height by boxing the corner?

@murda - yea! We're so sorry - as mentioned below, our commenting software wrecked havoc on that email trail below, which is why we are planning an update to this tutorial with illustrations. We've updated with what was the final edit in the trail - it reads: For each panel: Raw Width = Finished Width + Corner Depth + Seam Allowance x2. Raw Height = Finished Height + Half the Corner Depth + Seam Allowance x 1 ** Remember, your height will need to be added to in order to account for how you'll be finishing the TOP raw edge.

@ Barbara Ann -So sorry for the delay and I'm afraid our comment system deleted some of the question trail with all the back and forth. We believe it would be helpful to others to add the formula, however, since math scares a lot of people, we will need to create some diagrams to make things easier. In the meantime, you need to know your desired finished width and height, your desired finished corner depth and the seam allowance. For each panel: Raw Width = Finished Width + Corner Depth + Seam Allowance x2. Raw Height = Finished Height + Half the Corner Depth + Seam Allowance x 1 ** Remember, your height will need to be added to in order to account for how you'll be finishing the TOP raw edge. Say you want a 20” tall x 15” wide x 4” deep bag (dimensions without the top finished). RW = 15” + 4” + 1 (20”) and RH = 20” + 2” + .5” (22.5”).

Using your formula for the Raw Width/Height, when adding the Corner Depth (4), would I need to add any seam allowances for the box, for example would the cut-out be 4" or 4.5"

Also, in regards to Raw Height, if I want to add a 1/2" hem, would the RH formula change to include that number?

I know I have lots of questions, but I just want to get it right because I want to end up with specific sizes. I'm so excited at the idea of you creating a diagram, I'm a visual learner. Thanks so much.

@Barbra Ann - As you can see in the formula, we do indicate a seam allowance as part of the equation - so yes, you do add the seam allowance when figuring. Remember, as shown in the instructions above, your cut out is half the finished dimension, so if you want the 4" depth, you'd cut out a 2" box. The 1/2" seam allowance is accounted for -- you are working with a diagonal and then sewing 1/2" from the raw edge -- so all three seams are a consistant 1/2" (side seam, bottom seam, diagonal seam). The diagonal is wider than the cut corner... that's how it works. The best bet for you would be to try a test out of muslin, so you can better wrap your head around how the geometry works. Regarding the top finish, we mentioned that as well - that indeed, you do have to account for however you want to finish the top of the bag when calculating your raw height. We do have diagrams on our to-do list, but as it's a busy time of year, we can't promise when that will go live. Best of luck.

Thanks everyone for your help. Thanks for pointing out that "18" x 14" panels wtihout boxed corners would result in a 17" x 13" piece (not 16" x 13"). I don't know what I was thinking lol. I do have another question please. I have read conflicting instructions on the internet as far as including/not including the seam allowance line for the square cutout. If my desire is to end up with a 4" box bottom, do I cut out a 2"square the starts at the edge of the fabric or does it start on the seam line? I hope that makes sense. Thanks

@Barbara Ann - As you can see above, we are in the "count the seam allowance" camp when doing the cut out. The key is to keep everything consistant. If your seams coming into the corner are 1/2", your seam across the corner must also be 1/2".

I have been searching and searching for this info....THANKS for breaking down what adding the box cornerr does to the overall project. Would you mind giving further details about how much exactly does the box bottom reduce the height and width of the item. For example, If my front and back pieces were cut 18 tall by 14 wide, and I cut out squares on both sides that were 2" each. And I am using a 1/2" seam allowance...How would this reduce the dimensions of my pattern when finished....Without a boxed bottom it would be 16 tall by 13 wide, but if I added 2" squares what would happen? Thanks so much.

As always, the projects, ideas aand suggestions are very clear and helpful.What has helped me to get that 'ideal' box-corner is a series of templates I have cut from heavy cardboard. A series starting from a tiny 1/2 inch square for a pouch up to 2 1/2 inch square for a tote or larger bag.

Step one for me is to sew the side and bottom seams of outer and inner fabrics, not forgetting the space we leave for turninng.

Step two is to take my template, lay it on the seam and drawing a line around the two sides that are not sitting on the seam lines. I tend to use any pen, pencil as these lines will not show.

Step three is to line up the drawn lines with a 'dreaded' pin or two but also anchoring the sides of the 'triangle with a heavy paper clip. This allows me to withdraw the pin as I sew across the line and before I prick myself.

I have found that this helps me to get the seams lined up as well as leaving me room to correct or change the sizing if I make a mistake.

I think it's important to keep in mind that the larger your boxed corners, the more you are taking away from the height of the item you are making. So if you are creating your own pattern, make sure to allow for that

Once I got over my fear of boxed corners I grew to love doing them. One thing that is really helpfull -- cutting mats often have lines that cross to form a diagonal/triangle that you can use to ensure that your corners are as precises as possible. Aligning your peak/corner with the peak/corner on the board and ensuring that your edges align with the diagonal lines is very helpful. There are a lot of other ways to use this to make your best boxed corners (rulers, etc), but this is the starting point to work from.